While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
In organic electroluminescent devices, only 25% of electrons and holes recombine as singlet states while 75% recombine as triplet states according to simple spin statistics. Singlet and triplet states, and fluorescence, phosphorescence, and intersystem crossing are discussed in J. G. Calvert and J. N. Pitts, Jr., Photochemistry (Wiley, New York, 1966). Emission from triplet states is generally very weak for most organic compounds because the transition from triplet excited state to singlet ground state is spin-forbidden. Hence, many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence and thereby utilize only 25% of the electron and hole recombinations. However, it is possible for compounds with states possessing a strong spin-orbit coupling interaction to emit strongly from triplet excited states to the singlet ground state (phosphorescence). One such strongly phosphorescent compound is fac-tris(2-phenyl-pyridinato-N^C-)Iridium(III) (Ir(ppy)3) that emits green light (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H. -B. Bürgi, Inorg. Chem., 33, 545 (1994)). Organic electroluminescent devices having high efficiency have been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). Additional disclosures of phosphorescent materials and organic electroluminescent devices employing these materials are found in U.S. Pat. No. 6,303,238 B1, WO 00/57676, WO 00/70655 and WO 01/41512 A1.
Few organic-based emissive materials can be deposited as neat films. Usually it is necessary to codeposit them with a host material, either a charge transporting “small” molecule or a polymer, to get a reasonable light output. Well known host materials for dopant-host system include hole transporting 4,4′-N,N′-dicarbazole-biphenyl (CBP) and electron transporting aluminum tris(8-hydroxylquinoline) (Alq), which have been both used in OLEDs. However, the known host materials are not suitable host materials for all dopants. There continues to be a need in the art for suitable host materials for dopants which have short emission wavelength, such as in the green or blue regions of the spectrum.
In the emissive layer of phosphorescent OLEDs, the host is generally selected to have a triplet energy higher than that of the phosphorescent dopant in order to avoid exothermic energy quenching to the host. In addition, it is desirable for the host material to be an efficient charge carrier to achieve low drive voltage of the devices (Appl. Phys. Lett., 77, 904 (2000)). Host materials containing phosphineoxide have been disclosed in JP2003317965A and JP2004204140A. In both patent applications, the phosphineoxide group was used mainly as a linking group incorporating conventional charge (hole and electron) transporting units into one structure. The charge transporting ability of phosphineoxide itself has not been explored. Furthermore, the majority of the structures disclosed therein have triplet energies corresponding to emission in the red or deep red of the visible spectra. Publication JP2004204140A specifically requires that at least one naphthyl group be attached to the phosphorus atom (the triplet energy of naphthalene itself is 2.6 eV) consequently the triplet energy of the compounds should be no more than 2.6 eV. Therefore, there is a particular need in the art for host materials which can support dopants with green and blue phosphorescent emission.
It is a problem to be solved to provide a device containing a phosphorescent material in a light emitting layer, the device also containing a compound that enables high luminescent efficiency and low drive voltage.